Add security warning to Admin docs with dummy links

[vishaangelova]

This is a sub-issue of #1734, and the aim is to address the possibility of malicious dummy links in docs by adding a warning in the related Admin docs.

Parent issue description

For security reasons, we plan to replace any placeholder/fake URLs with non-hostable placeholder segments.

Remove segments that make a valid URL, including prefixes (http://) and TLDs (.com).

before:

<script src="https://<your-cdn-host>.com/path/to/elastic-apm-rum.umd.min-<version>.js" crossorigin></script>

after:

<script src="<your-url>/elastic-apm-rum.umd.min-<version>.js" crossorigin></script>

The placeholder <> can be adapted to use descriptive text for its purpose. For example, a placeholder for a splunk server might look like this:

server: <your-splunk-server-url>/<path>

[shainaraskas]

looking into alternatives in the parent issue

[shainaraskas]

ready